1. Field of the Invention
The present invention relates to a hydrogen generator configured to generate hydrogen-rich gas by steam-reforming a material at least containing carbon atoms and hydrogen atoms, and to a fuel cell system configured to generate electricity and heat using the hydrogen-rich gas.
2. Description of the Related Art
At present, a hydrogen gas supply system has not yet been developed as a general infrastructure. For this reason, some fuel cell systems, which have been developed and commercialized as a distributed power generation hydrogen generator, have a configuration in which a hydrogen gas generating means is provided along with a fuel cell, so that hydrogen gas, which serves as an electric-power generating fuel, generated by the hydrogen gas generating means is supplied to the fuel cell instead of directly supplying hydrogen gas to the hydrogen generator. For example, there is a fuel cell system in which a hydrogen generator configured to generate hydrogen using a material supplied from an existing infrastructure, such as city gas and LPG, is additionally provided.
Many of hydrogen generators generate hydrogen gas by causing a material such as city gas or LPG to undergo a chemical reaction (specifically, steam reforming) using a catalyst. With such hydrogen generators, when the hydrogen generator is repeatedly operated and stopped, the oxidization and reduction of the catalyst is more likely to be repeated at high temperatures than when the hydrogen generator is continuously operated, increasing the possibility of degrading the catalytic activity. In particular, at the stopping of the hydrogen generator, the catalyst within the hydrogen generator is in a high temperature state, and the probability of air coming into the interior of the hydrogen generator is increased because the internal pressure of the hydrogen generator is reduced due to the temperature decrease associated with the stop operation. When the catalyst is placed under a high-temperature oxidizing atmosphere, the catalytic activity of the catalyst noticeably decreases due to sintering or the like.
In order to prevent the decrease in the catalytic activity caused by the stop of the hydrogen generator, it is desirable to replace the generated gas remaining in the hydrogen generator using an inert gas such as nitrogen as a replacement gas (hereinafter, this is referred to as “internal gas replacement operation”). Nevertheless, as with hydrogen gas, an infrastructure for the inert gas has not been developed either; for this reason, it is desired to prevent decrease in the catalytic activity by performing the internal gas replacement operation without using an inert gas. There is an example of such a hydrogen generator in which the supply of a material and water is stopped in a stop operation after the temperature of the catalyst has been reduced, thereby preventing the catalyst from oxidizing even when air enters the interior of the hydrogen generator (for example, Japanese Unexamined Patent Publication No. 2000-290001).
There is another configuration of a hydrogen generator in which, with the temperature of the reformer being configured to be detectable, the temperature of the reformer is detected at a stop of a hydrogen generator and also an internal gas replacement operation is performed with a feed gas when the detected temperature becomes less than a predetermined temperature, thus preventing the air from entering. By performing the internal gas replacement operation with a feed gas, such a configuration makes it possible to prevent decrease in the activity of the shift catalyst particularly due to steam and oxygen (for example, see Japanese Unexamined Patent Publication No. 2000-95504.) Further, there are types in which, during a stop operation of the hydrogen generator, a gas mixture of a material and steam is flowed in the hydrogen generator to cool the catalyst naturally and an internal gas replacement operation is performed with air after the cooling (cf. Japanese Unexamined Patent Publication Nos. 2002-8701 and 2002-93447), and in which the internal gas replacement operation is performed with a material (for example, cf. Japanese Unexamined Patent Publication No. 2002-151124).
It should be noted that temperature conditions of a hydrogen generator at the stop vary depending on the operating state of the hydrogen generator until the stop. Herein, the “stop” refers to a time point at which a control signal for stopping is output from a controller, and a “stop operation period” refers to a period from when this signal is output to when the hydrogen generator completely stops.
For example, temperature conditions of a hydrogen generator at the stop vary between when the hydrogen generator is stopped after a long time operation and when the hydrogen generator is immediately stopped after the start-up. In addition, even when the hydrogen generator is stopped immediately after the starting of the hydrogen generator, there exist a state in which the interior of the hydrogen generator has not yet been heated sufficiently and a state in which it has been heated to a certain degree, depending on the state of the hydrogen generator before the start-up. For example, when the hydrogen generator having been stopped for a long time so that the temperature dropped to room temperature is started to operate and the hydrogen generator is stopped immediately thereafter, the interior of the hydrogen generator has not been heated sufficiently; on the other hand, when the hydrogen generator having been operated for a long time and kept at a high temperature is temporarily stopped, then restarted immediately thereafter, and stopped again, the interior of the hydrogen generator is in a heated condition to a certain degree in which the temperature is kept high.
As described above, temperature conditions of the hydrogen generator at the stop vary depending on the operation conditions that precede the stop, but the above-described conventional internal gas replacement operation is not necessarily adapted to such various temperature conditions of the hydrogen generators appropriately; moreover, failure to perform an appropriate replacement operation may cause the catalytic activity of the reforming catalyst to decrease. For example, the use of a gas that does not have an appropriate ratio of a material and water (steam) in the replacement operation may cause the catalyst to oxidize or cause the carbon in the material to deposit when the interior of the hydrogen generator is at a high temperature. On the other hand, if the interior of the hydrogen generator is at a low temperature, steam condenses inside the hydrogen generator, producing water. These also result in decrease in the catalytic activity of the reforming catalyst.